Why Does an LED Have to Be Connected to a Resistor?

Forward Conduction

• You can connect a power source to an LED in two ways: forward or reverse. The reverse, or backwards, connection puts the LED's anode to a negative voltage and the cathode to a positive voltage. Since the device is a diode, it blocks any current flow in this direction and produces no light. In the forward connection, the cathode has a negative voltage and the anode has positive voltage. Here, resistance is very low, current flows freely and the LED shines brightly.

Current Source

• Most power sources, such as batteries or AC adapters, are capable of between a few hundred milliamps to a few amps of current. When a circuit has low resistance, the source will give it as much current as it can. Resistance "slows down" the rate of current flow in a circuit, much as a narrow pipe accommodates only a slow flow of water.

Maximum Forward Current

• Many kinds of electronic devices, including LEDs, diodes and transistors, have a maximum design current. When current exceeds this limit, the device burns out and fails. Simple LEDs, used for indicators and numeric displays, have a limit of about 20 milliamps. High-power or high-brightness LEDs, such as those used for room lighting, have higher current limits, but you cannot exceed these, either, without burning out the device.

Ohm's Law

• A rule of electrical theory called Ohm's Law states that current in amps equals voltage in volts divided by resistance in ohms. For a given voltage, a low resistance allows greater current than a high one. An LED's forward resistance is low, so it needs additional resistance to keep it from burning out.

Series Resistor

• To limit the current flowing to an LED, use a resistor in a series connection, which means the LED's anode connects to one lead of a resistor, and the other resistor lead connects to positive power. The LED's cathode lead connects to negative power. Since the LED's leads are not both directly connected to power, electricity must flow through the resistor, which limits current. In a typical low-voltage direct current (DC) circuit, you can use a 470-ohm 1/4-watt resistor. To calculate an exact resistance, use the formula R = (Vs -- Vl) / I, where R is resistance in ohms, Vs is the supply voltage, Vl is the LED voltage, and I is the current. If the LED needs 2 volts and 20 milliamps of current, and you use a 9-volt battery, the formula becomes R = (9 -- 2) / .02, which works out to 350 ohms.